1. Field of the Invention
The present invention relates to an improvement of electrochromic display device of the type using an electrochemical redox reaction for the coloration and the bleaching reaction.
2. Prior Art
Recently, many kinds of an display devices which have been based on electrochemical phenomena have been developed and some of them are attracting attention in uses in the field of display. These device are generally called electrochromic display device and utilize electrochromism, namely, coloration and bleaching of a substance by means of the application of a voltage or current.
A typical construction of the electrochromic device comprises a cell having at least a transparent face and comprises a pair of electrodes and a liquid of an electrochromic substance in the cell, wherein by impression of predetermined electric signals across the electrodes, the colorations and bleachings are made and the state of the coloration lasts for a relatively long time period thereby attaining a memory function.
Generally speaking, the electrochromic substances are divided into solution type substances represented by a type using viologen compounds and dispersion type (solid-state) substances represented by a type using tungsten oxide. These two types are different in the physical nature of the coloration phenomena, but both uses use electrolyte substances. In order to obtain long life and stability of operation, the electrolytes should have physical, as well as, chemical stability. Especially, since the solution type electrochromic substance utilizes redox reactions for the coloration and bleaching reactions, the chemical stability of the liquid of electrochromic substance is strongly required.